Purpose. The primary purpose of this study was to determine how well instruction in agriculture and in biology could be integrated in a high school agriculture setting. This purpose was accomplished by testing the effect of the incorporation of biological principles instruction into a problem area in vocational agriculture on student achievement in agriculture and in applied biology. The effect of this incorporation on student attitudes was also measured.Methods. The population for this study consisted of all students in production vocational agriculture programs in Illinois. One hundred and four students from nine schools comprised the sample, 47 in the treatment group and 57 in the control group. Since random assignment of students to treatment groups was impossible, a quasi-experimental design was used. Treatments were randomly assigned to classes. A pretest was administered prior to the beginning of instruction, and a posttest and student attitude instruments were administered immediately following the completion of instruction. Pretest scores, student background categories, previous biology experience categories, post graduation plans, and attitudes toward agriculture were used as covariate measures to control for possible pretreatment group achievement differences. Data were analyzed by means of analysis of variance and covariance, t-tests, and correlational and descriptive statistics.Findings. (1) The overall achievement mean of students taught using the integrated approach was significantly higher than that of those taught using the traditional approach (p $<$.01). (2) The applied biology achievement mean of students taught using the integrated approach was significantly higher than that of those taught using the traditional approach (p $<$.01). (3) There were significant relationships between student achievement and background (r = $-.15$), previous biology experience (r =.41) and post graduation plans (r =.21). (4) There were significant differences in both overall achievement and in applied biology achievement between students from farm, rural nonfarm, and urban backgrounds (p $<$.05). (5) The attitude toward the curricular plan used of the students taught using the integrated approach was significantly more positive than that of those taught using the traditional approach (p $<$.001).